While I'm sure you could get an X under 310 Wh/mi if you drive conservatively in ideal conditions my experience is that it's nearly impossible if driving it in a natural and comfortable way (using climate control, accelerating reasonably, and keeping up with traffic).
I haven't yet reset my Trip A since taking delivery in the 25k miles I've driven my 75D over the past 15 months and my average is 411 Wh/mi according to the trips page
We have to keep in mind that this heavily influenced by my personal driving style, the terrain around me (mountainous foothills not too far from Seattle, WA, USA), and the general ambient temperature. I am typically on the heavy side of the pedal as I really enjoy the acceleration, I travel over a mountain pass a few times a month, I've towed trailers for at least 1,000 of the total miles which doubles consumption for those miles, and I've been in sub-freezing temperatures regularly over the past few months.
I keep the car set to "Energy" display rather than "Range" since the displayed estimates have never aligned to my experience. I have come to a general rule that I reliably get around 1.5 miles for each percent of the battery if I'm not flooring it. I charge to 90% daily and consistently get about 110 miles of range by time I'm down to a 25-30% charge.
Trip B from the picture is a one-way jaunt over the mountain pass I mentioned. The side I went up is steeper than the side I came down. It is primarily highway between 65 mph and 75 mph with AP engaged. When I take that same trip home I'll use roughly 7-10% less energy due to the gentler incline up and the increased regen going down the steep side.
Finally, I also notice that my use will spike to 800-1,000 Wh/mi initially but that's usually only for the first 1-3 miles due to the power needed to accelerate the car multiple times before getting out on to a road and having some proper distance for the calculation to average out over.